Three duplex modes, Frequency Division Duplex (FDD), Half-FDD (H-FDD) and Time Division Duplex (TDD), are currently supported by Long Term Evolution (LTE).
FDD refers to uplink transmission and downlink transmission in different carrier frequencies to allow concurrent reception and transmission of signals by a base station and a user equipment; and TDD refers to uplink transmission and downlink transmission in the same carrier frequency to allow transmission and reception of signals respectively by a base station and a user equipment in different periods of time, or allow reception and transmission of signals respectively by a base station and a user equipment in different periods of time.
In wireless communication system of LTE and earlier wireless communication systems, a cell is configured with only one carrier, and the maximum bandwidth in LTE system is 20 MHz, particularly as illustrated in FIG. 1.
Compared with the peak rate of LTE system, the peak rate of Long Term Evolution-Advanced (LTE-A) system improves significantly, wherein the peak rate of LTE-A system is required to be 1 Gbps in the downlink and 500 Mbps in the uplink. The requirement of the peak rate can not be met with only one carrier with a maximum bandwidth of 20 MHz. Thus, the bandwidth available to the user equipment in LTE-A system needs to be extended, and in view of this, Carrier Aggregation (CA) has been introduced, that is, a plurality of consecutive or inconsecutive carriers in the same base station (eNB) are aggregated together to serve the user equipment concurrently with a desirable rate. These carriers aggregated together are also referred to as Component Carriers (CCs). Each cell can be a component carrier, and cells (component carriers) in different eNBs can not be aggregated. In order to ensure that the user equipment of LTE system can operate over each of the aggregated carriers, the frequency of each aggregated carrier is no more than 20 MHz, particularly as illustrated in FIG. 2. There are four carriers that can be aggregated in the base station of LTE-A system as illustrated in FIG. 2 so that the base station can transmit data with the user equipment over the four carriers concurrently to thereby improve the throughput of the system.
At present carrier aggregation across systems is not supported by LTE system, that is, an FDD carrier can only be aggregated with an FDD carrier, and a TDD carrier can only be aggregated with a TDD carrier.
Both in the FDD mode and the TDD mode of LTE system, the time-length of a radio frame is 10 ms, and the time-length of a sub-frame is 1 ms. Seven TDD uplink-downlink configurations are defined for each radio frame of the TDD mode, particularly as illustrated in Table 1, wherein D represents a downlink (DL) sub-frame, U represents an uplink (UL) sub-frame, and S represents a special sub-frame.
TABLE 1(TDD uplink-downlink configurations)uplink-downlinkSub-frame indexconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
In FDD mode of LTE system, a user equipment receives downlink data in sub-frame n−4 and feeds back signaling of whether the data in the downlink sub-frame needs to be retransmitted, that is, feeds back Acknowledgement/Negative Acknowledgement (ACK/NACK) information in uplink sub-frame n, also referred to as a Physical Downlink Shared Channel Hybrid Automatic Repeat ReQuest (PDSCH HARQ) in uplink sub-frame n. When carriers are aggregated, ACK/NACK information corresponding to a plurality of downlink carriers in sub-frame n−4 will be fed back in uplink sub-frame n concurrently.
In TDD mode of LTE system, a user equipment may feed back, ACK/NACK information corresponding to a plurality of downlink sub-frames, in the same uplink sub-frame, that is, user equipment detects Physical Downlink Shared Channel (PDSCH) transmission or Physical Downlink Control Channel (PDCCH) indicating downlink semi-persistent scheduling release, in downlink sub-frame n-k and feeds back the corresponding ACK/NACK information in uplink sub-frame n, wherein kεK, and values in the set K depend upon the TDD uplink-downlink configuration of the system and a particular sub-frame index, particularly as illustrated in Table 2; and particularly special sub-frames of special sub-frame configuration 0 and 5 with a normal Cyclic Prefix (CP) and special sub-frames of special sub-frame configuration 0 and 4 with an extended CP have no ACK/NACK feedback, that is, the user equipment will not feed back ACK/NACK information for such special sub-frames.
TABLE 2(related TDD downlink K values: {k0, k1, Λ kM−1})uplink-downlinkSub-frame indexconfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8,———————7, 5, 4, 11, 66——775——77—
As illustrated in Table 2, a plurality of radio frames are arranged in order, that is, if the last sub-frame in radio frame a is k, then the first sub-frame in radio frame a+1 is k+1, and Table 2 takes a radio frame as an example and illustrates values of K corresponding to respective uplink sub-frames in the radio frame, wherein n−k<0 refers to a downlink sub-frame in the previous radio frame.
However, since carrier aggregation across systems is not supported by LTE system currently, there is no HARQ feedback solution for downlink data when carrier aggregation is performed among TDD system and FDD system so far.